Not completely sure, Cox (my provider) says in the small print:

"Static IP addresses may be required or dynamic IP addresses may be assigned without a static IP request, depending on location."

http://ww2.cox.com/business/arizona/data/pricing.cox

I don't take notice of my IP address much, so I can't say if they assigned a static IP address in my case. If they're filtering on IP addresses, I imagine that they get a lot of false positives.